The present invention relates to a distributed feedback semiconductor laser which is stably operable at a single wavelength.
Since an optical fiber exhibits a minimal transmission loss in a wavelength band of 1.5 to 1.6 .mu.m, this wavelength band is drawing attention as a wavelength band for long distance transmission. In this wavelenghth band, however, silica optical fibers have finite chromatic dispersion and, accordingly, it is necessary to employ a semiconductor laser which stably oscillates at a single wavelength even at a high speed modulation. On the other hand, an InGaAsP laser exhibits excellent characteristics using an InP substrate in this wavelength band, and a few distributed feedback lasers of this quaternary system have been fabricated.